This investigation was designed to study the late effects of fast neutrons with a mean energy of 15 MeV on the brain, spinal cord or lung of dogs, irradiated with the same fractionation schedule currently being used for cancer management in the Middle Atlantic Neutron Therapy Association project, compared with the late effects of megavoltage x-ray irradiation delivered in a clinical schedule. The late effects on the brain or spinal cord are being quantitatively evaluated by neurological exams and visual and sensory evoked responses through surface electrodes. Radiation changes in the lung are being monitored through studies of pulmonary mechanics and gas exchange as well as radionuclide studies. At the end of this study a quantitative histopathological evaluation of the irradiated organs will be done. One hundred fifty-two dogs have completed fractionated irradiation patterns with either photons or neutrons. Post-irradiation measurements of brain, cord and lung function have been performed every three months. Statistical analysis of radionuclide studies in the lung at both 3 and 6 months have shown that the relative biological effectiveness for changes in aerosol deposition, volume and perfusion in the lung are greater than 4 at 6000 rad of photons. Graphs of measurements of gas exchange in mechanical properties of the lung indicate that the RBE will also be greater than 4. Post-irradiation surface stimulation data are currently being evaluated. Neurological exams have been performed prior to necropsy. Moribund dogs have been euthanized, gross necropsies completed and tissues are being processed for qualitative scoring. BIBLIOGRAPHIC REFERENCES: Bradley, E.W., Late effects of neutrons in dogs. Particle Radiation Therapy: Proceedings of an International Workshop, October 1-3, 1975, Key Biscayne, Fla., 275-282, 1976.